(1) Field of the Invention
This invention relates to acoustical absorbing materials, and more particularly to acoustical absorbing material suitable for use with electronic equipment, such as, for example, a computer, which is required to show a good acoustical absorbing performance in a wide frequency range.
(2) Description of the Prior Art
Acoustical absorbing materials of the prior art usually consist of one layer of a porous sheet made of, for example, foamed plastics. The acoustical absorbing material of the type which consists of this single porous layer exhibits an acoustical absorbing characteristics which has a lower absorbing performance in a low frequency range but an absorbing performance increasing as the frequency increases. Therefore, if an increased acoustical absorbing effect should be desirable in the low frequency range, it has hitherto taken a measure to increase the thickness of the porous sheet.
However, the thickness of the acoustical absorbing material is limited in many case from the view point of using, therefore it is impossible to increase its thickness unlimitedly. Moreover, an increase in the thickness of the acoustical absorbing material incurs a rise in its cost, causing an increase in the production cost of electronic equipment and electrical household appliances.
In order to improve the acoustical absorbing performance in the low frequency range, there has been also provided such an acoustical absorbing material that porous sheets made of, for example, foamed plastics and high density sheets made of, for example, vinyl resins, are alternately superposed one over another. This type of acoustical absorbing material exhibits an acoustical absorbing performance superior in the low frequency range, but inferior in the high frequency range. To enhance the acoustical absorbing effect in the high frequency range, the thickness of the porous sheets may be increased. However, this would result in disadvantage similar to the acoustical absorbing material of the single porous sheet type, thereby such multilayer acoustical absorbing materials do not have a large flexibility in use.